Apparatus for coating articles by thermal evaporation



Oct. 31, 1950 M. N. LEWIS ETAL APPARATUS FOR comm; ARTICLES BY 'IHERIALEVAPORATION Filed Jan.

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Patented Oct. 31, 1950 APPARATUS FOR COATING ARTICLES BY THERMALEVAPORATION Margaret N. Lewis and Park H. Miller, Jr., Philadelphia,Pa., assignors to the United States of America as represented by theSecretary oi War Application January 3, 1946, Serial No. 638,875

1 Claim. 1

This invention relates in general to methods and apparatus for coating,and more particularly to methods and apparatus for coating b thermalvaporization.

Heretofore, the thermal vaporization in a vacuum of metals such asgermanium and silicon to produce a thin coating on a metal backingmember has been done by heating a small quantity of such metal in acrucible by means of an ordinary resistance heating element. Such amethod resulted in impurities in the film deposited on the backingmember. Such impurities, it was found, emanated from the cruciblematerial itself. For example, when a. quantity of silicon or germaniumwas placed in a tantalum crucible and directly heated by passing anelectric current through the crucible, it was found that the tantalumreacted with the silicon at the temperatures used. To prevent thisreaction, beryllium oxide was tried as a crucible and heated by atungsten wire coil. However, traces of beryllium were also found inspectographic pictures of the evaporated films.

In order to obviate the difliculties mentioned above, it was found thatthe metal to be vaporized in vacuo could be brought up to the requiredtemperature by means of direct electron bombardment so that theevaporated films were free of an discernible impurities due to reactionof the coating metal with the crucible.

It is an object of our invention to provide an apparatus and method forcoating by thermal vaporization in which the contamination of theevaporated film due to reaction between the coating metal and thecrucible is avoided.

It is another object of our invention to provide an apparatus and methodfor coating by thermal vaporization in which the coating material isbrought to its vaporization temperature while maintaining the crucibleat a much lower temperature.

It is another object of our invention to provide a. method of applying acoating in which the coating metal is vaporized in a vacuum by means ofdirect electron bombardment.

It is another object of our invention to provide an apparatus forcoating in which the coating metal is vaporized in a vacuum by means 01'direct electron bombardment.

The above and other objects and advantages will become readily apparentfrom the following specification when read in conjunction with theaccompanying drawing in which the single figure is a semi-diagrammaticview of one embodiment of our invention.

Referring to the drawing, it is seen that the (Cl. ill-12.2)

reference numeral l0 refers to a rigid base member of electricalinsulating material. A metal support or pedestal ll carries a cruciblel2 also made of metal, preferably tantalum. A quantity of the coatingmetal l3 which is to be vaporized is placed in the crucible II.

A cathode H which is to be a source of electrons for bombarding thecoating metal I3 is connected by means of the conductors l5 and I6 to asource or electrical energy IT. A focussing element l8, preferably ofcylindrical configuration is so situated as to focus the electronsemitted from the cathode H. In order to attract the electrons emittedfrom the cathode H to the metal I 3 to be vaporized, a, source ofelectrical energy i9 is provided, the positive side of which isconnected through conductor 20 to the metal table or support II. Thenegative side of the source of electrical energy 19 is connected to thenegative side of the cathode potential supply I".

The focussing element It is connected to the negative side of a sourceof electrical energy H by means of a conductor 22. The positive side ofthe source of electrical energy 2| is connected to the positive sourceof the source of electrical energy I 9.

The article to be coated indicated by the reference numeral 23 issupported upon a backing member 24 by any suitable means. The entireapparatus is enclosed by a transparent bell jar 25, the interior ofwhich may be evacuated by connecting a vacuum pump to the hoseconnection 28.

The degree of vacuum required for vaporization by electron bombardmentmust be such as to permit the establishment of a stead flow of electronsfrom the cathode to the coating metal H but need not be any higher than10- mm. of mercury. The high voltage supply I9 should be such .as tosupply a direct current voltage of between 3,500 to 5,000 volts at 10 to15 milliamperes. The negative potential at which the focussing elementi8 is to ,be placed should be sufficient to cause a narrow beamformation of electrons emitted from the cathode I4 so that as far aspossible substantially all of the electrons emitted from the cathodewill impinge upon the supply of coating metal l3.

Our invention avoids the possibility of contamination of evaporatedfilms due to reaction between the coating metal and the crucible. Bybringing the coating metal to the evaporating temperature andmaintaining the crucible at temperatures below the vaporizationtemperature 3 of the material forming the crucible, reaction between thecoating metal and the crucible does not occur, and contamination of theevaporated film is obviated.

While we have disclosed one embodiment oi our invention it is obviousthat many changes may be made without departing from the scope of theinvention as set forth in the appended claims.

Having thus disclosed our invention what is claimed is:

A device for coating an article by thermal evaporation comprising anevacuated chamber, supporting means within said chamber for saidarticle, an electrically conducting base member in the chamber, acontainer in the chamber for a coating metal to be vaporized, saidcontainer being supported by said base member, electron emitting meansincluding a cathode positioned intermediate said article supportingmeans and said electrically conducting member and in a straight linetherewith, a first source of direct current potential, means connectingthe positive terminal of said source of potential to said electricallyconducting base member and the negative terminal to said cathode: meansfor focusing the electrons from said cathode on said material to bevaporized, said means for focusing comprising a metal cylinder havingits axis on said straight line and positioned so as to surround thecathode, a second source of direct current potential and meansconnecting said second source between said focusing means and saidelectrically conducting bas member.

MARGARET N, LEWIS.

PARK H. MHLER, J R.

REFERENCES CITED The following references are of record in the tile oithis patent:

UNITED STATES PATENTS Number Name Date 1,326,794 Larsen Dec. 30, 19191,584,728 Case May 18, 1926 2,103,623 Kott Dec. 28, 1937 2,157,478Burkhardt, et a1 May 9, 1939 2,239,842 Burkhardt, et a1. Apr. 22, 19412,435,997 Bennett Feb. 17, 1948

